1). Field of the Invention
This invention relates generally to a semiconductor processing method, and more specifically to the manufacture of a lightly phosphorous doped silicon film that can be incorporated into a semiconductor device.
2). Discussion of Related Art
Integrated circuits are usually manufactured in and on silicon and other semiconductor substrates or wafers. Most often, silicon is used as the substrate for these integrated circuits. Silicon is also often used to fabricate the associated circuit elements. Circuit elements such as transistor gate electrodes, transistor gate dielectrics, and capacitor electrodes are made out of silicon. For example, a transistor typically has a gate dielectric film and a gate electrode, which is formed on the gate dielectric film, which is formed on a silicon substrate. The gate electrode is doped with a dopant such as boron, phosphorus, or arsenic. The substrate is subsequently heated to activate the dopant and make the electrode conductive.
Doping the circuit elements after the formation step increases cost. Insitu doping such as insitu phosphorous doped silicon films have been used in the making of many circuit elements, for example, in making gate electrodes, silicide stacks and floating gates. Insitu doping indicates that the films (or silicon films) are doped as they are being formed in the same process and in the same deposition chamber. Conventionally, silane or monosilane (SiH4) is normally used as a silicon source and phosphine (PH3) is used as dopant source for the insitu deposition of the phosphorous-doped silicon film.
In one embodiment, the fabrication of a phosphorous-doped silicon film is carried out in a conventional batch type chemical vapor deposition system. Such a system typically involves a hot wall furnace system, which includes a resistance furnace, a quartz reactor tube, some gas inlets, and a wafer boat that allows for processing of multiple wafers at the same time. Typically, multiple silicon wafers are vertically positioned upon the wafer boat for deposition. The wafers are radiantly heated by resistive heating coils surrounding the tube. Reactant gases are metered into one end of the tube (through some gas inlets using a mass flow controller). Reaction by-products are pumped out the other end of the tube (e.g., via an exhaust pump). Fabrication in batch limits the ability of varying the dopant concentration between wafers. It is also much more difficult to customize the film forming process for particular applications. Additionally, if anything goes wrong during the deposition process, a large batch of substrates are damaged and rendered useless.
One disadvantage of the conventional methods of insitu doping is that it is difficult to control dopant concentration especially in forming the lightly doped film. Another disadvantage is that the lightly doped film is not uniform.
Some current methods have used disilane (Si2H6) as the silicon source and phosphine (PH3) as the dopant source for the insitu deposition of the phosphorous-doped silicon film in a single-wafer deposition chamber. See for example, U.S. Pat. Nos. 5,607,724 and 5,614, 257, assigned to Applied Materials, Inc. These methods did not discuss the non-uniform problem for the phosphorous doping concentration in a lightly-doped silicon film.